The present invention relates to a process for evaluating the output signals of two pressure sensors of which a first sensor is designed for an upper pressure range, and a second sensor is designed for a lower pressure range. The two pressure ranges overlap in an intermediate range. While the pressures in the intermediate range are the same, the two sensors have different values of the output signal characteristics. The present invention further relates to a pressure measuring arrangement for carrying out the process as well as a pressure measuring head.
Various measuring principles are known for measuring pressures in the range from 1 bar absolute into the ultrahigh vacuum range. However, each vacuum measuring principle known today has disadvantages, the following being the most important disadvantages: limited measuring range; dependence on gas type; susceptibility to disturbing influences, such as temperature, contamination and aging; and cost factor.
A starting point for solving these problems, particularly also with respect to the limited measuring range, is the use and combination of several pressure sensors having different measuring principles which are acted upon by the same pressure medium. For this purpose, the following solutions are known.
In a first solution, the measuring signals of at least two separately constructed sensors which operate according to different measuring principles are processed on the same evaluating apparatus. Using the signal of one sensor, that is, when a limit pressure value of the pressure range assigned to this sensor is reached, the second sensor is locked-on. Depending on the pressure range, the signal of one or the other sensor is displayed on the evaluating measuring apparatus. Examples of this technology are the Pirani sensor/cold-cathode or hot-cathode ionization sensor combination of Balzer's PKG 100, TPG 300, IMG 300 model apparatuses, as well as, for example, the COMBIVAC, COMBITRON model apparatuses manufactured by Leybold Co.
An advantage of this first approach is the large measuring range. However, the high costs are disadvantageous, particularly as the result of providing two separate sensor arrangements and their mounting measures, for example, the use of two connecting flanges. Further, the output signal is neither constant nor single-valued over the pressure range due to switch over from one sensor to the other. Since the dependence on the gas type and other disturbing influences may be very different in the known different measuring principles, the change-over jumps may be very large. A stable pressure control, for example, is not possible in this change-over range.
In a second solution for reducing the cost and the space requirement, it is also known to combine different measuring sensors on a flange. The different sensors operate according to different measuring principles. However, then the evaluation of the sensor signals takes place as described above for the first solution. In this respect, reference can be made to the combination of a piezoresistive diaphragm-type sensor and a Pirani sensor on a flange, for example, in the case of the VSKP45M Model Apparatus manufactured by Thyracont Electronic GmbH, Passau, Germany. Also known is the combination of a hot-cathode ionization pressure gauge and a Pirani sensor on a flange, with respect to which reference is made to C. Edelmann, et al., Feingeratetechnik 24, Pages 408 to 411 (1975), as well as European Patent document EP-A-0 233 784 and U.S. Pat. No. 4,755,669, as well as to the Model AIG17P measuring tube manufactured by AML Co.
Under this aspect, the combination of a gas friction pressure gauge and a hot-cathode ionization pressure gauge on a flange is also known, with respect to which reference is made to U.S. Pat. No. 4,747,311. As mentioned above, in all these known cases, the sensor signals are changed-over when the range limits are reached, and their output signals are displayed only as an alternative. Nevertheless, costs are saved in comparison to the solution first described above.
A third known solution uses a combination of a gas friction pressure gauge and a Pirani sensor on a flange. In this combination, the two sensor output signals are processed such that an output signal is generated which is constant in an expanded pressure range without any change-over jumps. Reference is made in this respect to European Patent document EP-A-0 347 144.
Also known is a combination of a gas friction pressure gauge and a Pirani sensor on a flange and the combination of the two sensor signals in such a manner that an output signal is generated which is virtually independent of the gas type, with respect to which reference is made to European Patent document EP-A-0 379 841 corresponding to U.S. Pat. No. 4,995,264.
Furthermore, the combination of the signals of a diaphragm-type pressure sensor and a heat conduction sensor corresponding to a Pirani sensor is also known in order to develop altimeters which are sensitive in a large range. Such a combination is described in U.S. Pat. No. 3,064,478.
The present invention is based on the above-mentioned prior art known from European Patent document EP-A-0 347 144. According to this reference, a quartz oscillator is subjected to a vacuum and, similar to a Pirani sensor, the heat conduction into the environment from a heating spiral is detected by the heating effect on the quartz oscillator. In this case, the heating current of the spiral is controlled such that, when the pressure varies, the resonance frequency of the quartz oscillator remains constant. The electric heating energy supplied to the heating spiral as a control element is measured as one sensor output signal. The resonance voltage of the quartz oscillator is measured as the second sensor output signal. By recognizing that the pressure dependence of the power, fed for a constant resonance frequency of the heating spiral, decreases in a lower pressure range in order to move toward a constant minimal value, and that the resonance voltage does not decrease before an upper pressure range is reached, then by means of the summation of the two quantities to be considered, the sensor output signals, specifically the heating energy and the resonance voltage, determine a characteristic pressure discriminator curve which permits a pressure discrimination for seven pressure decades.
Although a large measuring range can be measured using the known art discussed in European Patent document EP-A-0 347 144, the suggested signal superposition technology is very specifically based on the thermal resonance behavior of a quartz oscillator.
There is therefore needed a process of the above-mentioned type in which, depending on the pressure range to be covered by the measuring technique, known pressure measuring sensors with their specific measuring ranges can be combined without generating measuring signal jumps or hysteresis effects at the transition range, that is, in the intermediate range. This has the purpose of permitting, depending on the usage, the selection of suitable sensor combinations which permit, in particular, increasing the pressure range measured one-to-one by the measuring technique using the targeted sensor selection, very significantly above that known from European Patent document EP-A-0 347 144. Further, the combination also particularly permits a massive enlarging of the one-to-one measuring range in comparison to an individual sensor.
These needs are met according to the present invention by a process for evaluating the output signals of two pressure sensors, of which a first sensor is designed for an upper pressure range, and a second sensor is designed for a lower pressure range. The two pressure ranges overlap in an intermediate range. While the pressures in the intermediate range are the same, the two sensors have different values on the output signal characteristics. When output signal characteristics exist in the intermediate range which are predominantly displaced in parallel, i.e., different merely by an offset value, a common output signal A is generated in the form: EQU A=fp.sub.p +(1-f)p.sub.k,
wherein:
p.sub.p is the output signal of the first sensor;
p.sub.k is the output signal of the second sensor; and
f is a weighting function which changes constantly monotonically with the pressure p. In the case that the signal characteristics predominantly differ with respect to their slopes, an output signal A is generated in the form: EQU A=p.sub.p.sup.f * p.sub.k.sup.(1-f),
wherein the weighting function f in the range at and below the upper limit pressure of the pressure range of the second sensor is selected to be essentially at 0, and in the range at and above the lower limit pressure of the pressure range of the first sensor, is selected to be essentially at 1.
A particularly preferred sensor combination according to the invention is that of a Pirani sensor and of a cold-cathode ionization sensor. Apart from the transition control from one sensor to the next first discussed here, the preferred above-mentioned combination, in comparison to the use of Pirani sensors with hot-cathode ionization sensors, has the significant advantage that the cold-cathode ionization sensor develops much lower heat than that of a hot-cathode ionization sensor. Thermal disturbances from one sensor to the next and disturbances as a result of waste gases are therefore considerably reduced. Thus, constructionally, the combination of a Pirani sensor and a cold-cathode ionization sensor can be much closer together than the constructional combination of a Pirani sensor and a hot-cathode ionization sensor.
Further, when considering the relevant measuring range of a Pirani sensor of below 10.sup.-3 mbar to above 100 mbar, as well as the relevant measuring range of a cold-cathode ionization sensor, as known, for example, from Max Wutz "Theorie und Praxis der vakuumtechnik" ("Theory and Practice of Vacuum Technology"), Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden, which ranges from at least 10.sup.-8 mbar to 10.sup.-2 mbar, it is found that, by means of the combination according to the invention, a pressure range can be covered which is much higher than seven decades and which passes through nine or more pressure decades, that is, for example, from 10.sup.-8 to above 100 mbar. Because of the suggested weighted combination of the sensor output signals, a one-to-one measuring takes place over the entire pressure range.
In the case of this preferred sensor combination, the weighting function is controlled by the output signal of the Pirani sensor which still provides relevant measurements in the entire pressure range, and particularly also in the intermediate pressure range above 10.sup.-4 mbar to 10.sup.-2 mbar, or, in the case of a preferred and optimum embodiment, to implement the above-mentioned weighting function by a combination of the Pirani output signal and a signal fed back from the weighted combined output signal.
In the case of a further advantageous embodiment, the implementation of the weighted signal superposition becomes easily possible particularly by using the analog switching technique on bipolar transistors.
Basically, the weighted signal superposition, as mentioned above, may take place by analog technology or, as will be explained in the following, by means of a pulse width modulation or, optionally, also by means of a computerized processing of digitized sensor output signals.
The multiplicative weighting, which is also used in the case of the Pirani sensor/cold-cathode ionization sensor combination preferred according to the invention, is preferably carried out by optimally controlling the weighting function by the combination of the Pirani output signal and a feedback signal which is derived from the weightedly combined output signal.
A pressure measuring sensor according to the invention, for satisfying the above-mentioned needs, includes two pressure measuring sensors of which a first sensor is designed for an upper pressure range, and a second sensor is designed for a lower pressure range. The pressure ranges overlap in an intermediate range. While the pressures are the same in the intermediate range, the sensors have different values of the output signal characteristics. The outputs of the sensors are guided to a weighting and evaluating unit whose output signal A follows at least in a first approximation: EQU A=fp.sub.p +(1-f)p.sub.k ;
or EQU A=p.sub.p.sup.f * p.sub.k.sup.(1-f) ;
wherein
A is an output signal of the weighting and evaluating unit;
p.sub.p is an output signal of the first sensor;
p.sub.k is an output signal of the second sensor; and
f is a weighting function which extends constantly monotonically in the intermediate range between 0 and 1.
A pressure measuring head according to the present invention has a pressure measuring arrangement basically comprising the above-mentioned preferred combination of a Pirani sensor and a cold-cathode ionization sensor which are constructionally combined on the head.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.